Transparent resins have been used for various kinds of optical materials using transparency equivalent to that of a mineral glass and impact property higher than that of a mineral glass. When one of transparent resins, a polythiourethane resin, is molded by casting polymerization and thus produced, a product cannot be obtained unless a resin is released from a mold. As known from the fact that a urethane resin is also used as an adhesive, the polythiourethane resin has been known as a resin having very strong adhesion so that it is essential to use a mold release agent for releasing the resin from the mold.
Examples of the mold release agent include an external mold release agent for coating a mold surface using a spray or the like and an internal mold release agent to be added to a raw material monomer in advance. There are problems in the external mold release agent such that its operation is not only troublesome, but also it is difficult to form a homogeneous release film and the surface precision is lowered. So, the internal mold release agent is preferably used.
Examples of the compound known as an internal mold release agent in the past include aliphatic alcohols, fatty acid esters, triglycerides, fluorine type surface active agents, higher fatty acid metal salts and the like. However, when these compounds were used, there have been defects such that it was difficult to be released, the surface or inside of the resin easily became turbid, and transparency of a resin by nature was easily deteriorated. In the field of optical materials including typical examples of plastic lenses requiring very high transparency, such deteriorated transparency becomes a very critical defect. Herein, as a means to solve the problem, various proposals have been made. For example, an internal mold release agent of an acidic phosphate ester compound and the like as described in Patent Documents 1 to 6 can be cited. In Patent Documents 1 to 3, there has been described that, as an additive other than an internal mold release agent, an organic tin compound such as dibutyltin dichloride and the like is necessarily added as a polymerization catalyst or a polymerization initiator.
By the way, as a catalyst used in the production reaction of a polythiourethane resin, organic tin type catalysts including typical examples of DBC (dibutyltin dichloride) and DBTDL (dibutyltin dilaurate) have been widely used from the past in view of the fact that the catalytic activity is high. However, problems have, in late years, been pointed out in these organic tin type catalysts from the viewpoints of toxicity and safety.
For example, tributyltin contained in DBTDL as impurities has a problem of the risk of injury to the human body as an environmental hormone. Furthermore, there has already been a movement to control the use of organic tin compounds in Europe taking the lead in this movement. Therefore, in the business world of spectacle lenses using polythiourethane resins, it has been urgently desired to develop a tin substituted catalyst. From now on, not only in Europe but also throughout the world, it is highly possible that the regulation on the use of organic tin catalysts is still more strengthened, while, in the polyurethane market, it becomes essential to develop a catalyst with high safety and high activity which can be a substitute of an organic tin catalyst.
To produce a polythiourethane resin as a material for plastic lenses, a casting polymerization method in which a polymerizable composition is generally injected into a mold for heat curing can be taken, whereas the polymerization reaction is carried out while gradually raising the temperature from low temperature to high temperature over several hours to several tens of hours. At that time, in order to obtain an optically homogeneous plastic lens, adding a catalyst is required to slowly carry out the polythiourethanization reaction controlled by heat lest thermal inhomogeneity be occurred while raising the temperature. Furthermore, in order to achieve full resin properties including optical properties, heat resistance and strength, it is necessary to complete the polymerization. In order to complete the polymerization, a method in which a catalyst with strong polymerization activity is used or an amount of the catalyst is increased can be cited. However, such a method has a problem such that the polymerization reaction has all been progressed while the prepared polymerizable composition is injected into a mold, that is, a sufficient pot life cannot be secured. Further, there is also a problem such that during the polymerization, exothermic heat is locally generated so that optical inhomogeneity is easily exhibited to lenses.
Patent Document 1: Japanese Patent Publication No. 1995-118989
Patent Document 2: Japanese Patent Publication No. 1994-20752
Patent Document 3: Japanese Patent No. 2842658
Patent Document 4: Japanese Patent Laid-open No. 1999-43493
Patent Document 5: Japanese Patent Laid-open No. 2000-281687
Patent Document 6: Japanese Patent Laid-open No. 2001-72692